1. Field of the Invention
The present invention relates to a method of and a system for controlling antilocking brakes employed in a vehicle, a motorcycle, etc.
2. Description of the Related Art
A system for controlling antilocking brakes has been employed in a vehicle, a motorcycle, etc., for example.
As a type of brake control system, there is known one in which a slip ratio of each wheel with respect to a road surface is computed based on the speed or velocity (Vc) of a running vehicle and the speed or velocity (Vw) of each wheel and the optimum braking is applied to the vehicle based on the computed slip ratio. Here, the slip ratio represents a value calculated from Vc-Vw/Vc.
The relationship between the slip ratio of each wheel with respect to the road surface and a friction coefficient (.mu.) relative to the direction of rotation of each wheel is normally represented as shown in FIG. 1. The slip ratio vs. friction coefficient characteristic varies depending on the state of the road surface due to rain, snow or sand or the like. However, the .mu. value shows the peak in the slip ratio range of 10% to 20%. Therefore, the braking control can be carried out by causing the slip ratio to converge to a range in the vicinity of the slip ratio range of 10% to 20%.
The relationship between a friction coefficient (.mu.) in the lateral or transverse direction (normal to the direction of the rotation) of each wheel and the slip ratio is then represented as shown in FIG. 2. In case of a road surface (hereinafter called a "low .mu. road") of a low friction coefficient, such as a frozen snow surface, the value of .mu. of the transverse direction of each wheel is reduced as compared with a road surface (hereinafter called a "high .mu. road") of a high friction coefficient, such as an asphalt road surface. It is therefore preferable to ensure the stability of the vehicle by setting a target slip ratio to a low value in the low .mu. road.
On the other hand, both a lock speed and a return or reset speed of each wheel with respect to the caliper pressure also vary greatly depending on .mu. of the road surface. In case of the high .mu. road, the lock speed is slow and the reset speed is fast. In case of the low .mu. road to the contrary, the reset speed is slow and the lock speed is fast (see FIG. 3). Thus, when the braking control is made at the same operation speed in varying the caliper pressure (hereinafter called a "pressure increasing and reducing rate") in spite of the fact that a difference is developed between the lock speed and the reset speed of each wheel with respect to the road surface owing to a variation in .mu., the optimum braking control cannot be attained for either a high .mu. road or a low .mu. road.
It is therefore considered that the braking control is carried out by estimating .mu. from an actual vehicle deceleration and a wheel velocity or the like. However, the value of .mu. tends to vary greatly due to a variation in the state of braking of a front wheel and/or a rear wheel, for example. It is thus considered that the caliper pressure is directly measured to more accurately estimate .mu.. However, an expensive and heavy hydraulic-pressure sensor is required, which will cause problems from the standpoint of the manufacturing cost and the weight. Therefore, this type of hydraulic-pressure sensor has not been conventionally used.